TMR Device with Novel Free Layer Structure

ABSTRACT

A composite free layer having a FL 1 /insertion/FL 2  configuration where a top surface of FL 1  is treated with a weak plasma etch is disclosed for achieving enhanced dR/R while maintaining low RA, and low λ in TMR or GMR sensors. The weak plasma etch removes less than about 0.2 Angstroms of FL 1  and is believed to modify surface structure and possibly increase surface energy. FL 1  may be CoFe, CoFe/CoFeB, or alloys thereof having a (+) λ value. FL 2  may be CoFe, NiFe, or alloys thereof having a (−) λ value. The thin insertion layer includes at least one magnetic element such as Co, Fe, and Ni, and at least one non-magnetic element. When CoFeBTa is selected as insertion layer, the CoFeB:Ta ratio is from 1:1 to 4:1.

This is a continuation of U.S. patent application Ser. No. 12/658,005,filed on Feb. 1, 2010; which is herein incorporated by reference in itsentirety, and assigned to a common assignee.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Patent ApplicationPublication 2009/0122450; U.S. Patent Application Publication2009/0121710; and U.S. Pat. No. 8,059,374; all assigned to a commonassignee, and which are herein incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to a high performance tunneling magnetoresistive(TMR) sensor in a read head and a method for making the same, and inparticular, to a first ferromagnetic layer (FL1) in a composite freelayer represented by a FL1/INS/FL2 configuration in which the FL1surface structure and surface energy are modified by a weak plasma etchtreatment before an insertion layer (INS) and second ferrormagneticlayer (FL2) are deposited thereby increasing the magnetoresistive (MR)ratio while maintaining strong coupling between the FL1 and FL2 magneticlayers.

BACKGROUND OF THE INVENTION

A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is akey component in magnetic devices such as Magnetic Random Access Memory(MRAM) and a magnetic recording head. A TMR sensor typically has a stackof layers with a configuration in which two ferromagnetic layers areseparated by a thin non-magnetic insulator layer. The sensor stack in aso-called bottom spin valve configuration is generally comprised of aseed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer,tunnel barrier layer, free layer, and capping layer that aresequentially formed on a substrate. The free layer serves as a sensinglayer that responds to external fields (media field) while the pinnedlayer is relatively fixed and functions as a reference layer. Theelectrical resistance through the tunnel barrier layer (insulator layer)varies with the relative orientation of the free layer moment comparedwith the reference layer moment and thereby converts magnetic signalsinto electrical signals. In a magnetic read head, the TMR sensor isformed between a bottom shield and a top shield. When a sense current ispassed from the top shield to the bottom shield (or top conductor tobottom conductor in a MRAM device) in a direction perpendicular to theplanes of the TMR layers (CPP designation), a lower resistance isdetected when the magnetization directions of the free and referencelayers are in a parallel state (“1” memory state) and a higherresistance is noted when they are in an anti-parallel state or “0”memory state. Alternatively, a TMR sensor may be configured as a currentin plane (CIP) structure which indicates the direction of the sensecurrent.

A giant magnetoresistive (GMR) head is another type of memory device. Inthis design, the insulator layer between the pinned layer and free layerin the TMR stack is replaced by a non-magnetic conductive layer such ascopper.

In the TMR stack, the pinned layer may have a syntheticanti-ferromagnetic (SyAF) configuration in which an outer pinned layeris magnetically coupled through a coupling layer to an inner pinnedlayer that contacts the tunnel barrier. The outer pinned layer has amagnetic moment that is fixed in a certain direction by exchangecoupling with the adjacent AFM layer which is magnetized in the samedirection. The tunnel barrier layer is so thin that a current through itcan be established by quantum mechanical tunneling of conductionelectrons.

A TMR sensor is currently the most promising candidate for replacing aGMR sensor in upcoming generations of magnetic recording heads. Anadvanced TMR sensor may have a cross-sectional area of about 0.1microns×0.1 microns at the air bearing surface (ABS) plane of the readhead. The advantage of a TMR sensor is that a substantially higher MRratio can be realized than for a GMR sensor. In addition to a high MRratio, a high performance TMR sensor requires a low areal resistance RA(area×resistance) value, a free layer with low magnetostriction (λ) andlow coercivity (Hc), a strong pinned layer, and low interlayer coupling(Hin) through the barrier layer. The MR ratio (also referred to as TMRratio) is dR/R where R is the minimum resistance of the TMR sensor anddR is the change in resistance observed by changing the magnetic stateof the free layer. A higher dR/R improves the readout speed. For highrecording density or high frequency applications, RA must be reduced toabout 1 to 3 ohm-um².

A MgO based MTJ is a very promising candidate for high frequencyrecording applications because its tunneling magnetoresistive (TMR)ratio is significantly higher than for AlOx or TiOx based MTJs asdemonstrated by S. Yuasa et al. in “Giant room-temperaturemagnetoresistance in single crystal Fe/MgO/Fe magnetic tunneljunctions”, Nature Materials, 3, 868-871 (2004), and in “Giant tunnelingmagnetoresistance up to 410% at room temperature in fully epitaxialCo/MgO/Co magnetic tunnel junctions with bcc Co(001) electrodes”, Appl.Phys. Lett., 89, 042505 (2006), and by S. Parkin et al. in “Gianttunneling magnetoresistance at room temperature with MgO (100) tunnelbarriers”, Nature Materials, 3, 862-867 (2004).

CoFeB has been used in the free layer for MgO based MTJs to achieve highMR ratio and a soft magnetic layer. D. Djayaprawira et al. showed thatMTJs with a CoFeB/MgO(001)/CoFeB structure made by conventionalsputtering can also have a very high MR ratio of 230% with advantages ofbetter flexibility and uniformity in “230% room temperaturemagnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, PhysicsLetters 86, 092502 (2005).

For a low RA application, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs canreach 138% at RA=2.4 ohm/μm² according to K. Tsunekawa et al. in “Gianttunneling magnetoresistance effect in low resistanceCoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read headapplications”, Applied Physics Letters 87, 072503 (2005). In this case,a DC-sputtered Mg layer was inserted between the CoFeB pinned layer andan RF-sputtered MgO layer, an idea initially proposed by T. Linn et al.in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode(CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure. Also, a Tagetter pre-sputtering prior to RF sputtering a MgO layer can achieve 55%TMR with 0.4 ohm/μm² as reported by Y. Nagamine et al. in “Ultralowresistance-area product of 0.4 ohm/μm² and high magnetoresistance above50% in CoFeB/MgO/CoFeB magnetic junctions”, Appl. Phys. Lett., 89,162507 (2006).

In order to achieve a smaller Hc but still maintain a high TMR ratio,the industry tends to use CoFeB as the free layer in a TMR sensor.Unfortunately, the magnetostriction (X) of a CoFeB free layer isconsiderably greater than the maximum acceptable value of about 5×10⁻⁶for high density memory applications. A free layer made of a CoFe/NiFecomposite has been employed instead of CoFeB because of its low λ andsoft magnetic properties. However, when using a CoFe/NiFe free layer,the TMR ratio will degrade. Another approach is a composite free layercontaining CoFeB with a positive λ and a NiFe layer with a negative λ toresult in a low λ and magnetic softness for the free layer. However, aCoFeB/NiFe type free layer structure is not usable because directcontact of CoFeB with NiFe will cause a drastic drop in the MR (TMR)ratio. Thus, an improved free layer in a TMR sensor is needed thatprovides low magnetostriction in combination with a high TMR ratio, lowRA value, and low coercivity.

U.S. Pat. No. 7,333,306 and U.S. Patent Application 2007/0047159 show atri-layered free layer represented by CoFe/CoFeB/NiFe to achieve lowcoercivity and low magnetostriction for either GMR-CPP or TMR sensors.

In U.S. Patent Application No. 2007/0139827, a free layer is describedthat includes a sense enhancing layer (Ta) sandwiched between a firstferromagnetic (FM) layer and a second FM layer. The first FM layer has apositive magnetostriction and is made of CoFeB or CoFe based alloyswhile the second FM layer has negative magnetostriction and is comprisedof CoFe, Ni, or NiFe based alloys.

U.S. Patent Application No. 2007/0188942 discloses a free layercomprised of three layers that include a lower NiFe or CoFe layer on thetunnel barrier layer, a Ta, Ru, Cu, or W spacer, and a CoFeB, CoFe, orNiFe upper layer.

U.S. Patent Application No. 2008/0061388 discloses a free layer with aCoFeB/Ru/CoFeTaB configuration.

U.S. Pat. No. 6,982,932 and U.S. Patent Application 2008/0152834describe free layers that are laminations of NiFe, CoFe, and CoFeB.

A MgO tunnel barrier is formed using a natural oxidation procedure asdescribed in U.S. Patent Application 2007/0111332.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a composite freelayer that simultaneously achieves low magnetostriction, low coercivity,and a high magnetoresistive (MR) ratio with low RA in a sensorstructure.

A second objective of the present invention is to provide a compositefree layer according to the first objective that may be incorporated ina CIP-GMR, CPP-GMR, or TMR sensor.

A further objective is to provide a method of forming a composite freelayer according to the first and second objectives that can be readilyimplemented in a manufacturing process and is cost effective.

According to one embodiment of the present invention, these objectivesare achieved by forming a TMR sensor on a suitable substrate such as abottom shield in a read head. The TMR sensor may have a bottom spinvalve configuration comprised of a seed layer, AFM layer, pinned layer,tunnel barrier layer, free layer, and capping layer which are formedsequentially on the bottom shield. The tunnel barrier layer ispreferably made of MgO, and the free layer has a composite structurerepresented by FL1(PT)/INS/FL2 where FL1 and FL2 are magnetic layersthat may be comprised of more than one material and INS is an insertionlayer which is preferably an alloy comprised of at least one magneticelement including Co, Fe, and Ni and at least one non-magnetic elementsuch as Ta, Ti, W, Zr, Hf, Nb, Mo, V, and Cr. B may be employed as anon-magnetic element in an INS having a quaternary alloy composition.The MR ratio is enhanced by performing a weak plasma etch treatment (PT)of the FL1 layer prior to depositing the insertion layer. In oneembodiment, the FL1 has a top surface comprised of CoFeB and less thanabout 0.2 Angstroms of the CoFeB surface is removed by the PT processcomprised of low power and high Ar pressure so as not to damage theunderlying tunnel barrier layer. Strong magnetic coupling between FL1and FL2 is maintained for high performance and good device stability.

In a second embodiment, the composite free layer described previously inthe TMR embodiment is formed in a GMR sensor that has a bottom spinvalve structure comprised of a seed layer, AFM layer, pinned layer,non-magnetic spacer, free layer, and capping layer. The GMR sensor mayhave a CIP or CPP configuration.

Typically, the stack of layers in the TMR or GMR sensor is laid down ina sputtering system. All of the layers may be deposited in the samesputter chamber but the plasma treatment is preferably performed in anoxidation chamber or oxidation/etch chamber within the same sputterdeposition mainframe. In the TMR embodiment, the MgO tunnel barrier ispreferably formed by depositing a first Mg layer on the pinned layerfollowed by a natural oxidation process on the first Mg layer to form aMgO layer and then depositing a second Mg layer on the MgO layer. Theoxidation step is performed in an oxidation chamber within thesputtering system. One or more annealing steps may be employed to setthe magnetization direction of the AFM and pinned layers. The TMR stackis patterned by a conventional method prior to forming a top shield onthe capping layer.

In yet another embodiment, the plasma treatment process may be appliedto a top spin valve configuration represented by seedlayer/FL2/INS/FL1(PT)/non-magnetic spacer/pinned/AFM/capping layer wherethe FL1 layer is treated with a weak plasma etch prior to depositing thenon-magnetic spacer. According to another top spin valve embodiment,both of the FL1 and FL2 layers may be treated with a weak plasma etch toimprove the crystal orientation of layers that are subsequentlydeposited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a TMR stack of layers with thetop surface of a first ferromagnetic layer (FL1) in a FL1/INS/FL2composite free layer of a bottom spin valve configuration that is beingtreated with a light etch.

FIG. 2 is a cross-sectional view of the partially formed TMR sensor inFIG. 1 after the insertion layer (INS) and second ferromagnetic layer(FL2) are sequentially deposited on the lightly etched FL1.

FIG. 3 shows a process flow diagram including a plasma etch treatmentfor forming a composite free layer according to one embodiment of thepresent invention.

FIG. 4 is a cross-sectional view showing a TMR stack of layers that hasbeen patterned to form a MTJ element during an intermediate step offabricating the TMR sensor according to one embodiment of the presentinvention.

FIG. 5 is a cross-sectional view of a TMR read head having a MTJ elementinterposed between a top shield and bottom shield and formed accordingto an embodiment of the present invention.

FIG. 6 is a cross-sectional view of a GMR read head having a compositefree layer according to a second embodiment of the present invention.

FIG. 7 is a cross-sectional view showing a TMR stack of layers with atop spin valve configuration wherein the top surface of a firstferromagnetic layer (FL1) in a FL2/INS/FL1 composite free layer wastreated with a weak plasma etch before depositing a non-magnetic spacerand other overlying layers.

FIG. 8 is a cross-sectional view showing a TMR stack of layers with atop spin valve configuration wherein the top surface of a secondferromagnetic layer (FL2) in a FL2/INS/FL1 composite free layer wastreated with a light plasma etch before depositing the insertion layerand FL1, and FL1 was treated with a light plasma etch before depositingoverlying layers.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a high performance magnetoresistive sensorhaving a composite free layer containing an insertion layer comprised ofat least one magnetic element and at least one non-magnetic elementformed between a first ferromagnetic layer (FL1) that has been lightlyplasma etched and a second ferromagnetic layer (FL2). A method isdisclosed for weakly etching FL1 and modifying its surface structurebefore depositing the insertion layer and FL2. While the exemplaryembodiment depicts a TMR sensor in a read head, the present inventionmay be employed in other devices based on a magnetoresistive elementsuch as a GMR-CPP or GMR-CIP sensor. The TMR or GMR sensor may have abottom spin valve, top spin valve, or multilayer spin valueconfiguration as appreciated by those skilled in the art.Magnetoresistive (MR) ratio may be used interchangeably with TMR ratiowhen referring to TMR sensors.

Referring to FIG. 1, a portion of a partially formed TMR sensor 1 of thepresent invention is shown from the plane of an air bearing surface(ABS). There is a substrate 10 that in one embodiment is a bottom leadotherwise known as a bottom shield (S1) which may be a NiFe layer about2 microns thick that is formed by a conventional method on asubstructure (not shown). Typically, a first gap layer (not shown) isformed on the bottom shield. It should be understood that thesubstructure may be comprised of a wafer made of AlTiC, for example.

A TMR stack is formed on the substrate 10 and in the exemplaryembodiment has a bottom spin valve configuration wherein a seed layer14, AFM layer 15, pinned layer 16, tunnel barrier layer 17, and apartially formed composite free layer that includes a firstferromagnetic layer FL1 18 a are sequentially formed on the substrate.Subsequently, as depicted in FIG. 2, the remainder of composite freelayer 18 is deposited followed by deposition of a capping layer 19 asthe uppermost layer in the TMR stack. The seed layer 14 may have athickness of 10 to 100 Angstroms and is preferably a Ta/Ru composite butTa, Ta/NiCr, Ta/Cu, Ta/Cr or other seed layer configurations may beemployed, instead. The seed layer 14 serves to promote a smooth anduniform grain structure in overlying layers. Above the seed layer 14 isan AFM layer 15 used to pin the magnetization direction of the overlyingpinned layer 16, and in particular, the outer portion or AP2 layer (notshown). The AFM layer 15 has a thickness from 40 to 300 Angstroms and ispreferably comprised of IrMn with a thickness between 40 and 70Angstroms. Optionally, one of PtMn, NiMn, OsMn, RuMn, RhMn, PdMn,RuRhMn, or MnPtPd may be selected as the AFM layer.

The pinned layer 16 preferably has a synthetic anti-parallel (SyAP)configuration represented by AP2/Ru/AP1 where a coupling layer made ofRu, Rh, or Ir, for example, is sandwiched between an AP2 layer and anAP1 layer (not shown). The AP2 layer which is also referred to as theouter pinned layer is formed on the AFM layer 15 and may be made of CoFewith a composition of about 10 to 50 atomic % Fe and with a thickness ofabout 10 to 50 Angstroms. The magnetic moment of the AP2 layer is pinnedin a direction anti-parallel to the magnetic moment of the AP1 layer.For example, the AP2 layer may have a magnetic moment oriented along the“+x” direction while the AP1 layer has a magnetic moment in the “−x”direction. A slight difference in thickness between the AP2 and AP1layers produces a small net magnetic moment for the pinned layer 16along the easy axis direction of the TMR sensor to be patterned in alater step. Exchange coupling between the AP2 layer and the AP1 layer isfacilitated by a coupling layer that is preferably comprised of Ru witha thickness from 3 to 10 Angstroms, and more preferably from 7 to 8Angstroms. The AP1 layer is also referred to as the inner pinned layerand may be a single layer or a composite layer. In one aspect, the AP1layer is amorphous in order to provide a more uniform surface on whichto form the tunnel barrier layer 17.

In the exemplary embodiment that features a bottom spin valveconfiguration, the tunnel barrier layer 17 is comprised of MgO because aMgO tunnel barrier is known to provide a higher TMR ratio than a TMRstack made with an AlOx or TiOx tunnel barrier. The MgO tunnel barrierlayer is preferably formed by depositing a first Mg layer having athickness between 4 and 14 Angstroms on the pinned layer 16, oxidizingthe Mg layer with a natural oxidation (NOX) process, and then depositinga second Mg layer with a thickness of 2 to 8 Angstroms on the oxidizedfirst Mg layer. The tunnel barrier is believed to have a MgO/Mgconfiguration immediately following deposition of the second Mg layer.The second Mg layer serves to protect the subsequently deposited freelayer from oxidation. However, during an annealing step followingdeposition of the entire TMR stack of layers, oxygen may diffuse fromthe lower MgO layer into the upper Mg layer. Therefore, the final tunnelbarrier layer composition is generally considered to be MgO. Note thatthe RA and MR ratio for the TMR sensor may be adjusted by varying thethickness of the two Mg layers in tunnel barrier layer 17 and by varyingthe natural oxidation time and pressure. A thicker MgO layer resultingfrom longer oxidation time and/or higher pressure would increase the RAvalue.

All layers in the TMR stack may be deposited in a DC sputtering chamberof a sputtering system such as an Anelva C-7100 sputter depositionsystem which includes ultra high vacuum DC magnetron sputter chamberswith multiple targets and at least one oxidation chamber. Typically, thesputter deposition process involves an argon sputter gas and a basepressure between 5×10⁻⁸ and 5×10⁻⁹ torr. A lower pressure enables moreuniform films to be deposited.

The NOX process may be performed in an oxidation chamber within thesputter deposition system by applying an oxygen pressure of 0.1 mTorr to1 Torr for about 15 to 300 seconds. In the exemplary embodiment, noheating or cooling is applied to the oxidation chamber during the NOXprocess. Oxygen pressure between 10⁻⁶ and 1 Torr is preferred for anoxidation time mentioned above in order to achieve a RA in the range of0.5 to 5 ohm-um². A mixture of O₂ with other inert gases such as Ar, Kr,or Xe may also be used for better control of the oxidation process.

The present invention anticipates that a MgO barrier layer 17 could beformed by depositing a MgO layer on a pinned layer with a rf-sputteringor reactive sputtering method. It should be understood that theperformance of a TMR sensor fabricated with a barrier layer comprised ofsputtered MgO will not be as desirable as one made according to thepreferred embodiment of this invention. For example, the inventors haveobserved that the final RA uniformity (1σ) of 0.6 um circular devices ismore than 10% when the MgO tunnel barrier layer is rf-sputtered and lessthan 3% when the MgO tunnel barrier is formed by DC sputtering a Mglayer followed by a NOX process.

Optionally, other materials such as TiOx, AlTiO, MgZnO, Al₂O₃, ZnO,ZrOx, or HfOx, or any combination of the aforementioned materialsincluding MgO may be used as the tunnel barrier layer 17.

Previously, we disclosed composite free layer structures that result inTMR or GMR sensors having high TMR ratio, low Hc with low λ, and a lowRA value. For example, in related U.S. Patent Application Publication2009/0121710, we described a CoFeB/non-magnetic/NiFe trilayerconfiguration for a free layer where the non-magnetic layer is made ofHf, V, Zr, Nb, Ta, Mo, or Cr. High TMR ratio is achieved because CoFeBand NiFe layers are separated by a Ta insertion layer, for example.CoFeB and NiFe are magnetically coupled through orange-peel typecoupling which tends to align the magnetic moments in parallelconfiguration. However, this coupling is relatively weak and in a realdevice has to compete with magnetostatic coupling from the edge of thetwo ferromagnetic free layers which tends to align the magnetic momentsof the CoFeB and NiFe layers anti-parallel. Moreover, stress inducedanisotropy tends to align the moments for CoFeB and NiFe perpendicularto one another because of the opposite signs of magnetostriction. As aresult, magnetic noise for this free layer structure is rather high.Although signal amplitude is high, improvement in signal to noise ratio(SNR) is limited and a further enhancement in SNR is desirable.

In related U.S. Patent Application Publication 2009/0122450, wedemonstrated that a free layer with a trilayer configuration comprisedof at least one CoFe layer having a (+) λ value and at least onenegative λ layer such as CoB or FeB can yield a high performance sensor.Furthermore, in related patent application HT08-035, a FeCoBTa or CoTalayer is inserted between two ferromagnetic layers in a composite freelayer to achieve high dR/R, low magnetoresistance, and a low RA value.Here we disclose another discovery that provides enhanced dR/R and maybe incorporated in the aforementioned free layer configurations. Inparticular, the first ferromagnetic layer (FL1) in a composite freelayer 18 represented by FL1/INS/FL2 may be modified by a weak etchtreatment to generate a modified FL1 18 m prior to depositing theinsertion layer (INS) 18 b and second ferromagnetic layer (FL2) 18 c.

Returning to FIG. 1, an important feature of the present invention isthe composite free layer 18 formed on the tunnel barrier layer 17.Tunnel barrier 17 has a first surface in contact with pinned layer 16and a second surface contacting free layer 18 where both first andsecond surfaces are formed parallel to the substrate 10. In oneembodiment, the free layer 18 has a FL1/INS/FL2 configuration whereinthe FL1 layer may be a composite of two or more layers preferablyincluding at least one CoFeB layer to result in a high dR/R ratioassociated with a MgO tunnel barrier layer and a free layer comprised ofCoFeB. FL1 18 a has an inner surface that contacts the second surface ofthe tunnel barrier layer 17. There is also an outer surface 18 topposite the inner surface. Preferably, FL1 18 a is ferromagnetic andhas a composition represented by Co_(W)Fe_((100-W)) or[Co_(W)Fe_((100-W))]_((100-y))B_(Y) where w is from 0 to about 100% andy is from 10 atomic % to about 40 atomic %, or the FL1 layer may be analloy of one of the aforementioned compositions comprised of one or moreadditional elements such as Ni, Ta, Mn, Ti, W, Zr, Hf, Tb, Mg, and Nb.The FL1 layer 18 a typically has a positive λ value and may be a singlelayer or a composite with multiple layers having a thickness from 2 to40 Angstroms that is relied upon to provide a high dR/R ratio. Forexample, when one of CoFeB, CoFe/CoFeB, or CoFe/CoFeB/CoFe is selectedas FL1 layer 18 a, the dR/R (TMR ratio) is advantageously increased.When FL1 layer 18 a is a composite of two or more layers includingCoFeB, the thickness of the CoFeB layer is preferably greater than theother layer or layers in the composite in order to maximize dR/R. Notethat a CoFeB layer generally has a large (+) λ value which must beoffset by a (−) λ value in one or more other layers in the free layer 18in order to achieve a magnetostriction less than about 5×10⁻⁶ for highperformance.

In the exemplary embodiment, FL1 18 a is deposited and then treated witha weak plasma etch treatment (PT) indicated by reactive ions 40 thatimpinge on the outer surface 18 t. Preferably, the PT process comprisesa weak plasma etch resulting in an etch rate of less than about 0.01Angstroms/second and removal of less than about 0.2 Angstroms FL1 18 ato generate a modified FL1 18 m (FIG. 2). In one aspect, the plasma etchconditions comprise a high Ar pressure of >1 E-2 (0.01) Torr and a lowpower of less than 20 Watts for a period of about 10 to 150 seconds. Itshould be understood that other inert gases such as Ne, Kr, or Xe may beemployed for the PT process and the <0.2 Angstrom film loss is estimatedfrom a separate experiment in which a light plasma etch is performed fora substantially longer period of time so that a thickness loss ofsignificantly greater than 1 Angstrom can be measured with reasonablecertainty. For example, the thickness loss of a CoFeB film after beingsubjected to a light plasma etch process for an hour can be divided by60 to determine the approximate film loss after a one minute etch usingthe same conditions.

Although not bound by theory, it is believed that the weak PT processresults in an enhanced dR/R for the TMR sensor because of one or both ofthe following conditions: (1) a modified surface structure of FL1 18 mwith respect to other portions of the first ferromagnetic layer; and (2)increased surface energy of FL1 18 m to create nucleation sites andpromote smooth and conformal growth in subsequently deposited insertionlayer 18 b, FL2 18 c, and capping layer 19. When FL1 18 a comprises asingle CoFeB layer or a composite with an uppermost CoFeB layer, it isbelieved that the weak PT process helps CoFeB crystallization along apreferred orientation (100) during a subsequent annealing step and mayfurther block the impact of a NiFe FL2 layer on the CoFeB crystalstructure. Only a weak plasma etch treatment is used since a strong PTcould penetrate FL1 18 a and damage tunnel barrier layer 17. Preferably,the PT process is performed in an oxidation chamber or in a chamber thatis acceptable for etching in a sputter deposition mainframe. Therefore,the wafer flow to form a composite free layer 18 during TMR devicefabrication according to the present invention comprises formation ofFL1 in a sputter deposition chamber, plasma treatment of FL1 in aseparate “etch-capable” chamber as described above, and then depositionof INS 18 b and FL2 18 c in a sputter deposition chamber where all ofthe aforementioned processes occur within the same mainframe.

The insertion layer 18 b is an alloy made of at least one magneticelement and at least one non-magnetic element and is deposited on themodified outer surface of FL1 18 m shown as FL1(PT) in FIG. 2. The atleast one magnetic element is selected from Fe, Co, and Ni, and the atleast one non-magnetic element is selected from Ta, Ti, W, Zr, Hf, Nb,Mo, V, Mg, and Cr. Boron may be employed as a non-magnetic element in aninsertion layer comprised of a quaternary alloy. Insertion layer 18 bpreferably has an overall non-magnetic character with a thickness from 2to 10 Angstroms. By keeping the insertion layer 18 b thickness constantand increasing the magnetic element content, the coupling strengthbetween the FL1 and FL2 layers is increased but TMR ratio is decreased.Lowering the magnetic element content in the insertion layer 18 b willhave the opposite effect. A strong coupling (Hcp) between the FL1 layer18 a and FL2 layer 18 c is desirable in order to minimize noise in thesensor and improve the signal to noise (SNR) ratio. Moreover, magneticstability improves as the Hcp value increases. Note that insertion layer18 b has a first side that interfaces with the outer surface 18 t ofFL1(PT) 18 m and a second (opposite) side that adjoins FL2 layer 18 c.

The FL2 layer 18 c is also ferromagnetic and may be a single layer or acomposite having two or more layers and a thickness from 2 to 50Angstroms. In one aspect, the FL2 layer 18 c is comprised of aCo_(W)Fe_((100-W)) layer where w is from 0 to about 100 atomic %, aNi_(Z)Fe_((100-Z)) layer where z is from about 70% to 100%, or acombination of the aforementioned layers. Furthermore, the embodimentencompasses a FL2 layer 18 c that is an alloy wherein CoFe or NiFe arecombined with one or more elements selected from Ni, Ta, Mn, Ti, W, Zr,Hf, Tb, Nb, or B. For example, the FL2 layer 18 c may have a CoFe, NiFe,or CoFe/NiFe/CoFe composition. In an embodiment wherein CoFe/NiFe/CoFeis selected as the FL2 layer 18 c, the NiFe layer is preferablysubstantially thicker than the CoFe layers in order to take advantage ofthe soft magnetic character of NiFe and the (−) λ contribution from aNiFe material to offset the (+) λ value in modified FL1 layer 18 m. In aFL2 layer 18 c, a NiFe layer may be sandwiched between two CoFe layersto avoid the NiFe layer contacting an insertion layer containing Tasince adjoining Ta and NiFe layers are known to form a so called “deadzone” that degrades dR/R. In general, FL2 materials or compositestructures having a (−) value and that promote strong coupling throughthe insertion layer 18 b with modified FL1 layer 18 m are preferred.However, FL2 layer 18 c may have a small (+) λ value when used incombination with a FL1 layer 18 m having a small (+) value so that anoverall λ value of <5×10⁻⁶ can be achieved for free layer 18.

With regard to insertion layer 18 b, it should be understood that analloy of a magnetic element and a non-magnetic element may be formed byco-sputtering a target of each element in a sputter deposition chamber.When multiple elements are involved as in a CoFeBTa alloy, for example,a CoFeB target may be co-sputtered with a Ta target to achieve thedesired composition in the resulting CoFeBTa insertion layer. In apreferred embodiment, the ratio of CoFeB:Ta in the CoFeBTa alloy isabout 2:1 to give an overall non-magnetic character for the insertionlayer 18 b. However, the CoFeB:Ta ratio may vary from about 1:1 to 4:1and still retain the advantages provided by the composite free layer 18of the present invention. In general, a CoFeB composition in the CoFeBTaalloy is preferred where Co is from about 40 to 70 atomic %, Fe is from20 to 40 atomic %, and B is from 10 to 30 atomic %. In another preferredembodiment, CoTa is selected as the insertion layer 18 b and has a Co:Taratio of about 2:1. Optionally, the Co content may be varied between 50and 80 atomic % to afford a Co:Ta ratio between 1:1 and 4:1 and anoverall non-magnetic character. As stated previously, increasing thecontent of the magnetic element which is Co in this example willincrease the coupling strength between the FL1 and FL2 layers but willlower the TMR ratio. Decreasing the Co content to near 50 atomic % willlower the coupling strength but will increase the TMR ratio.

In one embodiment, the capping layer 19 formed on FL2 layer 18 c may becomprised of Ru, Ta, or a combination of Ru and Ta. Optionally, thecapping layer 19 may be made of other materials used in the art.

Referring to FIG. 3, a process flow diagram is provided to furtherdescribe the formation of a composite free layer 18 according to thepresent invention. In step 100, a FL1 18 a is deposited on anon-magnetic spacer such as a tunnel barrier layer in a TMR sensor or ona metallic spacer in a GMR sensor. In one embodiment, all steps 100-103are performed within the same sputter deposition mainframe tool toimprove throughput. Unlike steps 100, 102, and 103 which occur insputter deposition chamber, step 101 comprises the PT process and isperformed in an oxidation chamber or in an etch chamber that is capableof generating a light plasma etch condition. In step 102, the insertionlayer 18 b is deposited on the modified FL1 18 m resulting from step101. Finally, FL2 18 c is deposited on insertion layer 18 b in step 103.It should be understood that all layers 14-19 within the TMR sensor maybe formed in the same sputter deposition main frame. However, the tunnelbarrier layer 17 is preferably oxidized in an oxidation chamber to avoidcontaminating a sputter deposition chamber with oxygen. It is importantto keep oxygen out of the free layer 18 so as not to degrade magneticproperties and decrease the magnetoresistance ratio.

Once the TMR stack is complete, the partially formed read head 1 may beannealed in a vacuum oven within the range of 240° C. to 440° C. with anapplied magnetic field of at least 2000 Oe, and preferably >8000 Oe, forabout 30 minutes to 10 hours to set the pinned layer and free layermagnetization directions. The present invention also anticipates thatthe anneal process may include at least two steps as we have previouslydisclosed in U.S. Patent Application Publication 2009/0229111. It shouldbe understood that under certain conditions, depending upon the time andtemperature involved in the anneal process, the tunnel barrier layer 17may become a uniform MgO tunnel barrier layer as unreacted oxygen in thelower MgO layer diffuses into the adjacent Mg layer in the MgO/Mg stack.

Referring to FIG. 4, the TMR stack is patterned by following aconventional process sequence. For example, a photoresist layer 20 maybe coated on the capping layer 19. After the photoresist layer 20 ispatterned, a reactive ion etch (RIE), ion beam etch (IBE), or the likeis used to remove underlying layers in the TMR stack that are exposed byopenings in the photoresist layer. The etch process may stop on thefirst gap layer above the bottom shield 10 or between the bottom shieldand a barrier layer (not shown) to give a TMR sensor with a top surface19 a and sidewalls 21.

Referring to FIG. 5, an insulating layer 22 may be deposited along thesidewalls 21 of the TMR sensor. The photoresist layer 20 is then removedby a lift off process. A top lead otherwise known as a top shield 25 isthen deposited on the insulating layer 22 and top surface 19 a of theTMR sensor. Similar to the bottom shield 10, the top shield 25 may alsobe a NiFe layer about 2 microns thick. The TMR read head 1 may befurther comprised of a second gap layer (not shown) formed between thetop surface 19 a and the top shield 25.

In a second embodiment represented by FIG. 6 that relates to a GMR-CPPor GMR-CIP device, the magnetoresistive element 1 is a stack of layerswhich is the same as described previously except the tunnel barrierlayer 17 is replaced by a non-magnetic spacer layer 27 that may be Cu,for example. The modified FL1 in composite free layer 18 contacts a topsurface of the non-magnetic spacer 27. The magnetoresistive element 1may be patterned by the same sequence of steps described previously withrespect to FIG. 4. Alternatively, the non-magnetic spacer in a GMR CPPdevice may have a confining current path (CCP) configuration wherein aninsulating layer such as a metal oxide having metal paths formed thereinis sandwiched between a first metal layer and a second metal layer. Forexample, the first and second metal layers may be Cu and an AlOx layerwith Cu paths therein may be formed between the two Cu layers asunderstood by those skilled in the art.

Referring to FIG. 7, the present invention also encompasses anembodiment where the TMR (or GMR) sensor has a top spin valveconfiguration. The same set of materials and processes are used aspreviously described with respect to the bottom spin valve embodiments.In one aspect, the top spin valve configuration may be represented byseed/FL2/INS/FL1(PT)/non-magnetic spacer/pinned/AFM/capping layer. A keyfeature is that a top (inner) surface of FL1 has been treated with alight plasma etch to form FL1(PT) 18 m prior to formation ofnon-magnetic spacer 27, and other overlying layers. The outer surface ofFL1 18 m is still considered to be the side contacting the insertionlayer 18 b whereas the inner surface of FL1 is the side contacting thenon-magnetic spacer 27. Similar to the previous embodiments, it isbelieved that the light plasma etch treatment described previouslymodifies the surface of a ferromagnetic layer which is FL1. Preferably,FL1 comprises CoFeB. In an embodiment with a composite FL1 layer, thereis preferably an uppermost CoFeB layer in the FL1 stack so that thelight plasma etch will have the largest impact in modifying surfacetexture and/or modifying surface energy to promote a favorable crystalorientation in subsequently deposited layers. However, PT treatment ofNiFe, CoFe, or alloys thereof which are formed as the uppermost FL1layer is also expected to provide a benefit in terms of improvedmagnetic properties such as a higher magnetoresistive ratio.

In yet another top spin valve embodiment illustrated in FIG. 8, the TMR(or GMR) sensor in FIG. 7 may be further modified by treating the FL2layer 18 c with a light plasma etch to generate a FL2(PT) 18 d layerprior to depositing the insertion layer 18 b and overlying layers. Thus,FL2 18 c may be treated with a first light plasma etch to form FL2(PT)18 d, and subsequently following deposition of INS 18 b and FL1 18 a,the FL1 layer may be treated with a second light plasma etch to produceFL1(PT) 18 m. Thus, the inner surface of FL2 18 d is believed to haveone or both of a modified structure or increased surface energy withrespect to other portions of the FL2. As indicated earlier, thepreferred embodiment is where FL1 is comprised of CoFeB or where FL1 isa composite with an uppermost CoFeB layer. However, CoFe or an alloythereof as the uppermost layer in a composite FL1 stack is also expectedto generate an improvement in MR ratio while maintaining otheracceptable magnetic properties in the TMR (or GMR) sensor.

Comparative Example 1

An experiment was conducted to demonstrate the improved performanceachieved by implementing a composite free layer in a TMR sensoraccording to the present invention. Three TMR stacks hereafter referredto as Wafer A, Wafer B, and Wafer C shown in Table 1 were fabricated onsix inch AITiC wafers. Wafer A represents a TMR structure as previouslydisclosed in related application HT08-035 in which CoFeBTa is employedas an insertion layer between two ferromagnetic layers in a compositefree layer. Wafer B is a TMR stack formed according to an embodiment ofthe present invention and is similar to Wafer A except the FL1 in theFL1/INS/FL2 composite free layer is modified with a PT process beforethe insertion layer and FL2 are deposited. Wafer C is the same as WaferA except a thin CoFe layer is inserted between a CoFeB FL1 layer and theinsertion layer to improve the MR ratio. In all three examples, the TMRstacks have a composition represented byTa/Ru/IrMn/CoFe/Ru/CoFe/MgO/free layer/capping layer. Ta/Ru is acomposite seed layer wherein both Ta and Ru layers have a 20 Angstromthickness. The AFM layer is IrMn and has a 70 Angstrom thickness. Thepinned layer has an AP2/Ru/AP1 structure in which the AP2 layer is a 25Angstrom thick Co₇₅Fe₂₅ layer, the Ru coupling layer has a 7.5 Angstromthickness, and the AP1 layer is a 25 Angstrom thick Co₇₅Fe₂₅ layer. TheMgO tunnel barrier was formed by depositing a 7 Angstrom thick lower Mglayer that was subjected to a NOX process before a 3 Angstrom thickupper Mg layer was deposited. The capping layer in this example is Ru/Tawherein the lower Ru layer is 10 Angstroms thick and the upper Ta layeris 60 Angstroms thick. The free layer has a FL1/insertion layer/FL2configuration in which FL1 is Co₉₀Fe₁₀ 3/Co₆₀Fe₂₀B₂₀10 and FL2 isNi₉₀Fe₁₀30 where the numbers following CoFe, NiFe, and CoFeB indicatethe thickness of that particular layer. The CoFeBTa insertion layer is 6Angstroms thick in all examples. TMR stacks were formed on a NiFe shieldand were annealed under vacuum at 280° C. for 5 hours with an appliedfield of 8000 Oe before patterning to form 0.8 micron circular devices.All wafers listed in Table 1 have reasonably good magnetic softnessincluding coercivity around 4 to 5 Oe and λ of approximately 1×10⁻⁶.

TABLE 1 Magnetic properties of TMR sensors withTa/Ru/IrMn/CoFe/Ru/CoFe/MgO/free/cap configuration Wafer MR ratio IDFree Layer Structure RA dR/R ave. A CoFe3/CoFeB10/CoFeBTa6/NiFe30 1.058.5% B CoFe3/CoFeB10/PT/CoFeBTa6/NiFe30 1.0 63.7% CCoFe3/CoFeB10/CoFe3/CoFeBTa6/NiFe30 1.0 61.0%

As shown in Table 1, a PT process applied to the FL1 layer(CoFe3/CoFeB10) in Wafer B can increase the MR ratio by 9% relative towafer A. As indicated earlier, the enhanced MR ratio is understood toresult from a modification of the CoFeB surface by the weak plasma etchtreatment in a manner that helps CoFeB crystallization along a preferredorientation and by increasing surface energy and thereby creatingnucleation sites. Although an additional CoFe layer in Wafer C alsogenerates a higher MR ratio, this alternative is not desirable becauseof a thicker TMR stack and a higher Bs which means a weaker effectivecoupling field (Hcp) between FL1 and FL2.

All samples in Table 1 demonstrate that a high dR/R and low RA can beachieved with a CoFeBTa insertion layer formed between two ferromagneticlayers. However, Wafer B which includes a PT process applied to the FL1provides the best overall performance and is more desirable forproducing advanced TMR (and GMR) sensors. Furthermore, a high Hcp isachieved by selecting a 6 Angstrom thick film of CoFeBTa as theinsertion layer. The CoFeB:Ta composition ratio is about 2:1 and thesingle layer of CoFeBTa is non-magnetic at this composition. As statedearlier, by decreasing the insertion layer thickness, dR/R decreases butHcp increases. On the other hand, increasing the insertion layerthickness above 6 Angstroms (not shown) would increase dR/R but lowerthe Hcp value. Further optimization of dR/R and Hcp is possible byadjusting the magnetic:non-magnetic element ratio in the insertionlayer. Moreover, λ for the composite free layer may be adjusted byvarying the composition or thickness of the NiFe layer with minimaleffect on dR/R. All samples listed in Table 1 have reasonably goodmagnetic softness with a coercivity (Hc) around 4 to 5 Oe and λ of about1×10⁻⁶. Magnetic properties were obtained from 0.8 micron circulardevices. It should be understood that the dR/R values will increase asthe sensor size decreases.

The advantages of the present invention are that a high TMR ratio ofgreater than 60% can be achieved simultaneously with a low RA value ofabout 1 ohm-um² and low magnetostriction of approximately 1×10⁻⁶ whichis a significant improvement over the prior art. Furthermore, softmagnetic properties (low Hc) are realized with a compositeFL1(PT)/INS/FL2 free layer composition as disclosed herein. The PTprocess may be implemented with a minimal effect on device cost ormanufacturing throughput since the weak plasma etch may be performed inthe same sputter deposition mainframe in which the TMR or GMR sensorstack is formed. A low temperature anneal process may be employed whichis compatible with the processes for making GMR sensors.

While this invention has been particularly shown and described withreference to, the preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

1. A method of forming a magnetoresistive element in a TMR sensor,comprising: (a) forming a seed layer on a substrate; (b) depositing afirst ferromagnetic layer (FL2) on the seed layer, said composite freelayer has an inner surface contacting the seed layer, an outer surfacefacing away from the seed layer, and is comprised of one or more ofCo_(W)Fe_((100-W)) where w is from 0 to about 100 atomic %,Ni_(Z)Fe_((100-Z)) where z is from about 70% to 100%, and an alloywherein CoFe or NiFe are combined with one or more elements selectedfrom Ni, Ta, Mn, Ti, W, Zr, Hf, Tb, Nb, and B; (c) performing a weakplasma etch treatment (PT) on the outer surface of said firstferromagnetic layer (FL2) thereby forming a modified FL2 that has one orboth of a modified surface structure and increased surface energy withrespect to other portions of the FL2 layer; (d) sequentially depositingan insertion layer (INS) and a second ferromagnetic layer (FL1) on themodified surface of the first ferromagnetic layer (FL2) to give acomposite free layer with a FL2(PT)/INS/FL1 configuration, saidinsertion layer has at least one magnetic element selected from Fe, Co,and Ni and at least one non-magnetic element selected from Ta, Ti, W,Zr, Hf, Nb, Mo, V, Mg, and Cr, and said second ferromagnetic layer iscomprised of one or more of Co_(W)Fe_((100-W)),[Co_(W)Fe_((100-W))]_((100-Y))B_(Y) where w is from 0 to about 100% andy is from 10 atomic % to about 40 atomic %, and an alloy of one of theaforementioned compositions comprised of one or more additional elementsincluding Ni, Ta, Mn, Ti, W, Zr, Hf, Tb, and Nb; (e) sequentiallyforming a non-magnetic spacer, pinned layer, and antiferromagnetic (AFM)layer on the FL1 layer; and (f) forming a capping layer on the AFMlayer.
 2. The method of claim 1 wherein the non-magnetic spacer is atunnel barrier layer that is comprised of one or more of MgO, MgZnO,ZnO, Al₂O₃, TiOx, AlTiO, HfOx and ZrOx.
 3. The method of claim 2 whereinformation of the MgO tunnel barrier layer is comprised of depositing afirst Mg layer, performing a natural oxidation process to form a MgOlayer, and then depositing a second Mg layer on the MgO layer.
 4. Themethod of claim 1 further comprised of performing an annealing stepafter step (f) wherein the annealing conditions comprise a temperaturebetween about 240° C. and 340° C. with an applied magnetic field of atleast 2000 Oe for about 2 to 10 hours.
 5. The method of claim 1 whereinthe second ferromagnetic layer has a thickness from about 2 to 40Angstroms and is comprised CoFeB, CoFe/CoFeB, or CoFe/CoFeB/CoFe.
 6. Themethod of claim 1 wherein the insertion layer has a thickness from about2 to 10 Angstroms and has a CoFeBTa composition wherein the ratio ofCoFeB to Ta is from about 1:1 to 4:1.
 7. The method of claim 1 whereinthe insertion layer has a CoTa composition wherein the ratio of Co to Tais from about 1:1 to 4:1.
 8. The method of claim 1 wherein the firstferromagnetic layer has a thickness from about 2 to 50 Angstroms and iscomprised of CoFe, NiFe, or CoFe/NiFe/CoFe.
 9. The method of claim 1wherein the weak plasma etch comprises Ar or another inert gas at apressure greater than about 0.01 Torr and with a power less than about20 Watts.
 10. The method of claim 1 further comprised of performing aweak plasma etch treatment (PT) on a top surface of the secondferromagnetic layer (FL1) prior to depositing the non-magnetic spacer.